Stabilizing prosthesis support structure

ABSTRACT

A tibial support structure includes a platform portion and a medullary portion that are monolithically formed as a single piece. The medullary and platform portions of the augment component are adapted to accommodate and mechanically attach to a tibial baseplate, and are individually shaped and sized to replace damaged bone stock both within the tibia, as well at the tibial proximal surface. The monolithic formation of the tibial support structure provides a strong and stable foundation for a tibial baseplate and facilitates restoration of the anatomic joint line, even where substantial resections of the proximal tibia have been made. The tibial support structure may be made of a bone-ingrowth material which facilitates preservation and rebuilding of the proximal tibia after implantation, while also preserving the restored joint line by allowing revision surgeries to be performed without removal of the tibial support structure.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit under Title 35, U.S.C. §119(e) ofU.S. Provisional Patent Application Ser. No. 61/488,549, filed May 20,2011 and entitled STABILIZING PROSTHESIS SUPPORT STRUCTURE, the entiredisclosure of which is hereby expressly incorporated herein byreference.

BACKGROUND

1. Technical Field

The present disclosure relates to orthopaedic prostheses, and moreparticularly, to stabilized tibial support structures for use with aknee prosthesis.

2. Description of the Related Art

Orthopaedic prostheses are commonly utilized to repair and/or replacedamaged bone and tissue in the human body. For example, a kneeprosthesis may be used to restore natural knee function by repairingdamaged or diseased articular surfaces of the femur and/or tibia. Kneeprostheses may include a femoral component implanted on the distal endof the femur, which articulates with a tibial component implanted on theproximal end of a tibia to replicate the function of a healthy naturalknee.

One goal of knee replacement procedures is to reproduce or enhance thekinematics of the natural knee using the associated prostheticcomponents. More generally, such procedures seek to achieve kinematiccharacteristics that promote favorable patient outcomes such asminimized pain, proper joint function through a wide range of motion,and the longest possible prosthesis service life.

One aspect of establishing proper kinematics in a knee joint prosthesisis replication of the healthy natural “joint line” of the knee, i.e.,the line spanning the medial and lateral points of contact between thefemoral condyles and abutting tibial articular surfaces. To ensure thatthe natural joint line is preserved in the joint replacement procedure,the distal portion of the femur and the proximal portion of the tibiamay each be resected by an amount corresponding to the thicknesses ofthe femoral and tibial components, respectively, such that the effectiveoverall lengths of the femur and tibia remain unchanged afterimplantation of the prosthetic components.

However, in some cases the proximal tibia or distal femur may havesevere degeneration, trauma, or other pathology which necessitatesresection of more bone than can be compensated for by traditionalfemoral and tibial components. In such cases, augments may be used toeffectively increase the thickness of the implanted component, therebycompensating for the additional thickness of the bone resection.Alternatively, a thicker prosthetic component can be employed instead ofa component/augment combination.

In the proximal tibia, poor quality bone stock may also exist in thediaphyseal and/or metaphyseal region within the tibia. In such cases, asurgeon may opt for a second kind of augment, such as an augment havinga generally cone-shaped outer profile corresponding to the generallycone-shaped bone defect typically encountered within the tibia.Exemplary tibial cone augments are disclosed in U.S. patent applicationSer. No. 11/560,276, filed Nov. 15, 2006 and entitled PROSTHETIC IMPLANTSUPPORT STRUCTURE, and in U.S. patent application Ser. No. 12/886,297,filed Sep. 20, 2010 and entitled TIBIAL AUGMENTS FOR USE WITH KNEE JOINTPROSTHESES, METHOD OF IMPLANTING THE TIBIAL AUGMENT, AND ASSOCIATEDTOOLS, both commonly assigned with the present application, the entiredisclosures of which are hereby expressly incorporated by referenceherein.

Where particularly acute degeneration of the proximal tibial bone stockhas occurred, both a “cone” type augment and a “platform” type augmentmay be needed to i) replace resected bone stock within the tibia and ii)provide an elevated platform for a tibial baseplate component,respectively. In such cases, one or both of the augments may be cementedin place using bone cement, which adheres selected prosthetic kneecomponents to one another and to the surrounding healthy bone stock.This bone cement may also be used join the pair of augments to oneanother, and to the tibial baseplate.

In some instances, such as where a knee prosthesis is implanted in ayounger patient, a revision surgery may eventually become necessary torepair or replace damaged or worn out prosthesis components. Suchrevision surgery may require the removal and/or replacement of thetibial baseplate, which if cemented in place would typically be removedtogether with any augment components used in the previous surgery. Boneingrowth into the material of the augment components may have occurredduring the service life of the original prosthesis, possiblynecessitating removal of additional healthy bone from the proximal tibiain order to fully dislodge the ingrown augment components.

SUMMARY

The present disclosure provides a tibial support structure that includesa platform portion and a medullary portion that are monolithicallyformed as a single piece. The medullary and platform portions of theaugment component are adapted to accommodate and mechanically attach toa tibial baseplate, and are individually shaped and sized to replacedamaged bone stock both within the medullar region of the tibia, as wellat the tibial proximal surface. The monolithic formation of the tibialsupport structure provides a strong and stable foundation for a tibialbaseplate and facilitates restoration of the anatomic joint line, evenwhere substantial resections of the proximal tibia have been made. Thetibial support structure may be made of a bone-ingrowth material whichfacilitates preservation and rebuilding of the proximal tibia afterimplantation, while also preserving the restored joint line by allowingrevision surgeries to be performed without removal of the tibial supportstructure.

Advantageously, the tibial support structure may be implanted withoutthe use of bone cement. The support structure/bone interface may besecured through the use of a porous bone ingrowth material on the outersurface of the support structure, such as highly porous tantalummaterial made in accordance with Trabecular Metal® technology availablefrom Zimmer, Inc. of Warsaw, Ind. (Trabecular Metal® is a trademark ofZimmer, Inc.). The support structure/baseplate interface may be securedby mechanical attachment, such as through the use of fasteners. Thiscementless securement procedure facilitates future revision proceduresby establishing a secure foundation for the tibial baseplate upon theproximal tibia, comprised of the support structure and ingrown bone,while also allowing the tibial baseplate to be mechanically disconnectedfrom the support structure in the event of a revision surgery.

In one form thereof, the present disclosure provides a support structurefor use in conjunction with a prosthesis component, the supportstructure comprising: a platform having a proximal surface and a distalsurface defining a platform thickness therebetween, the proximal surfaceand the distal surface cooperating to define a platform outer peripheryshaped to correspond with a periphery of a resected proximal tibia, theplatform outer periphery defining a platform medial-lateral width and aplatform anteroposterior length; and a medullary portion extendingdistally from the distal surface of the platform, the medullary portionmonolithically formed with the platform and comprising: a medullaryportion anteroposterior diameter less than the platform anteroposteriorlength; a medullary portion medial-lateral diameter less than theplatform medial-lateral width; and a medullary portion height measuredalong a proximal/distal extent of the medullary portion.

In another form thereof, the present disclosure provides a supportstructure for use in conjunction with a prosthesis component, thesupport structure comprising: a platform having a proximal surface and adistal surface defining a platform thickness therebetween, the proximalsurface and the distal surface cooperating to define a platform outerperiphery shaped to correspond with a periphery of a resected proximaltibia, the periphery divided into a medial side and an opposing lateralside, the platform outer periphery defining a platform medial-lateralwidth and a platform anteroposterior length; and a medullary portionextending distally from the distal surface of the platform and from atleast one of the medial side and the lateral side, the medullary portionmonolithically formed with the platform and comprising: a medullaryportion anteroposterior diameter less than the platform anteroposteriorlength; a medullary portion medial-lateral diameter; and a medullaryportion height measured along a proximal/distal extent of the medullaryportion.

In yet another form thereof, the present disclosure provides a supportstructure kit comprising: a first nominal size support structurecomprising: a first platform having a proximal surface and a distalsurface defining a first platform thickness therebetween, the proximalsurface and the distal surface of the first platform cooperating todefine a first platform outer periphery shaped to correspond with aperiphery of a first resected proximal tibia, the first platform outerperiphery divided into a medial side and an opposing lateral side, theplatform outer periphery defining a first platform medial-lateral widthand a first platform anteroposterior length; and a first medullaryportion extending distally from the distal surface of the platform andfrom at least one of the medial side and the lateral side, the medullaryportion monolithically formed with the platform and comprising: a firstmedullary portion anteroposterior diameter less than the first platformanteroposterior length; a first medullary portion medial-lateraldiameter; and a first medullary portion height measured along aproximal/distal extent of the first medullary portion; and a secondnominal size support structure larger than the first nominal sizesupport structure, the second nominal size support structure comprising:a second platform having a proximal surface and a distal surfacedefining a second platform thickness therebetween, the proximal surfaceand the distal surface of the second platform cooperating to define asecond platform outer periphery shaped to correspond with a periphery ofa second resected proximal tibia, the second platform outer peripherydivided into a medial side and an opposing lateral side, the platformouter periphery defining a second platform medial-lateral width and asecond platform anteroposterior length; and a second medullary portionextending distally from the distal surface of the platform and from atleast one of the medial side and the lateral side, the medullary portionmonolithically formed with the platform and comprising: a secondmedullary portion anteroposterior diameter less than the second platformanteroposterior length; a second medullary portion medial-lateraldiameter; and a second medullary portion height measured along aproximal/distal extent of the medullary portion; at least one of thefirst platform medial-lateral width, the first platform anteroposteriorlength, the first medullary portion anteroposterior diameter, the firstmedullary portion medial-lateral diameter, and the first medullaryportion height smaller than a corresponding one of the second platformmedial-lateral width, the second platform anteroposterior length, thesecond medullary portion anteroposterior diameter, the second medullaryportion medial-lateral diameter, and the second medullary portionheight.

The present disclosure provides a monolithic implant support structurewhich provides a stable implant mounting surface in a severely damagedor diseased bone. In the exemplary embodiments discussed below, thesupport structure provides a foundation for supporting a tibialbaseplate that is resistant to subsidence while also facilitatingreplacement and/or augmentation of metaphyseal or diaphyseal bone withinthe tibia. The support structure may be made of a porous bone ingrowthmaterial that provides a scaffold for bone ingrowth on multiplesurfaces. These surfaces present large, three-dimensional areas of boneingrowth material to the surrounding healthy bone for secure and stablelong term fixation of the support structure to the proximal tibia. Atibial baseplate may be mechanically attached to the support structure,which facilitates later removal of the tibial baseplate during arevision surgery while preserving the prosthesis foundation provided bythe support structure and ingrown bone.

A support structure in accordance with the present disclosure may beformed from a single piece of highly porous biomaterial. A highly porousbiomaterial is useful as a bone substitute and as cell and tissuereceptive material. A highly porous biomaterial may have a porosity aslow as 55%, 65%, or 75% or as high as 80%, 85%, or 90%, or may have anyporosity within any range defined by any of the foregoing values. Anexample of such a material is produced using Trabecular Metal®Technology generally available from Zimmer, Inc., of Warsaw, Ind.Trabecular Metal® is a trademark of Zimmer, Inc. Such a material may beformed from a reticulated vitreous carbon foam substrate which isinfiltrated and coated with a biocompatible metal, such as tantalum, bya chemical vapor deposition (“CVD”) process in the manner disclosed indetail in U.S. Pat. No. 5,282,861 to Kaplan, the entire disclosure ofwhich is expressly incorporated herein by reference. In addition totantalum, other metals such as niobium, or alloys of tantalum andniobium with one another or with other metals may also be used.

Generally, the porous tantalum structure includes a large plurality ofligaments defining open spaces therebetween, with each ligamentgenerally including a carbon core covered by a thin film of metal suchas tantalum, for example. The open spaces between the ligaments form amatrix of continuous channels having no dead ends, such that growth ofcancellous bone through the porous tantalum structure is uninhibited.The porous tantalum may include up to 75%, 85%, or more void spacetherein. Thus, porous tantalum is a lightweight, strong porous structurewhich is substantially uniform and consistent in composition, andclosely resembles the structure of natural cancellous bone, therebyproviding a matrix into which cancellous bone may grow to providefixation of the support structure to the patient's bone.

The porous tantalum structure may be made in a variety of densities inorder to selectively tailor the structure for particular applications.In particular, as discussed in the above-incorporated U.S. Pat. No.5,282,861, the porous tantalum may be fabricated to virtually anydesired porosity and pore size, and can thus be matched with thesurrounding natural bone in order to provide an improved matrix for boneingrowth and mineralization.

The support structure may be formed from bone ingrowth material, such asporous tantalum as described above, which provides a scaffold for theingrowth and interdigitation of bone with both the platform and themedullary portion of the support structure. As such ingrowth occurs overtime, the support structure becomes integrally formed with the tibia toprovide a stable, bone-like support foundation for a tibial baseplate.Advantageously, as noted below, this support foundation may remain inplace even through a revision surgery to replace a tibial baseplate witha new tibial baseplate.

Generally, a small size support structure is adapted for a small sizetibia and a relatively small medullary defect within such tibia, whichis filled in by the medullary portion of the structure. A larger sizesupport structure, on the other hand, is adapted for a larger tibiahaving a relatively large volume of defective bone within the tibia.However, it is contemplated that any size platform may be paired withany size medullary portion. In an exemplary embodiment, a family or kitof support structures may be provided with differing support structuresize/geometry combinations. Each individual support structure may besuitable for one of a wide range of natural tibia sizes and bone defectgeometries.

Support structures according to the invention may be used to restore thejoint line of the natural knee where a large amount of the proximaltibia has been resected to remove correspondingly large amounts ofdiseased, damaged or otherwise defective bone stock. The combination ofplatforms into a single monolithic structure with medullary portions,ensures that this joint line is maintained over a long period of time byproviding a large bone-contacting surface area. In addition, thismonolithic combination presents many bone-contacting faces, each ofwhich are oriented in a different direction with respect to the othersto yield a “3-dimensional” or multi-faceted profile of bone-contactingfaces. This 3-dimensional profile facilitates multidirectionalstabilization of the support structure, and of the tibial baseplatemounted thereto, thereby minimizing or eliminating subsidence,anteroposterior movement and medial-lateral movement of the tibialprosthesis in vivo. Moreover, it has been found that the stabilityprovided by a monolithic support structure made in accordance with thepresent disclosure provides greater stability than would otherwise beprovided by a separate tibial cone-shaped implant and a plate-shapedtibial augment implant, whether used in combination or alone.

Advantageously, a support structure made in accordance with the presentdisclosure does not require the use of cement for fixation to a bone.This lack of cement facilitates bone ingrowth by allowing bone tointerdigitate more deeply with the porous bone contacting surfaces ofthe platform. This deep bone ingrowth provides stronger and more securefixation than could be expected from adhesion between bone cement andbone. Thus, a support structure in accordance with the presentdisclosure provides a bone replacement and restoration mechanism whichgives rise to a stable, bone-like support structure for tibial baseplatecomponents and other associated knee prosthesis components.

Also advantageously, the tibial baseplates are removable from theirsupport structures in a revision surgery, even if substantial boneingrowth has occurred between the tibia and support structures. Becauseno cement is required, as discussed above, cemented fixation between atibial baseplate and a support structure in accordance with the presentdisclosure is not required. Rather, mechanical fixation may be used,such as with a fastener and a nut. If a revision surgery is required,such mechanical fixation can be reversed by removing fastener from thenut, thereby freeing the tibial baseplate from the support structure.The support structure can be left behind, and may therefore remainthoroughly interdigitated with ingrown bone. This remaining supportstructure obviates the need for removal of any further bone stock duringa revision surgery, and provides a reusable, stable and strong supportplatform for a new tibial baseplate and/or other knee prosthesiscomponents.

Further, the strength of fixation between a support structure inaccordance with the present disclosure and the adjacent bone isunexpectedly stronger than other designs adapted for use without bonecement. The monolithic, integral nature of the support structure resultsin a stronger implant as compared to two separate implants separatelyaffixed to the bone. Thus, the overall area of bone ingrowth for thesupport structures is substantially larger than any other similarlysized individual tibial augment structure.

BRIEF DESCRIPTION OF THE DRAWINGS

The above mentioned and other features and advantages of thisdisclosure, and the manner of attaining them, will become more apparentand the invention itself will be better understood by reference to thefollowing description of embodiments of the invention taken inconjunction with the accompanying drawings, wherein:

FIG. 1A is a perspective view of a relatively small-sized tibialbaseplate with a baseplate support structure made in accordance with thepresent disclosure attached thereto;

FIG. 1B is another perspective view of the support structure shown inFIG. 1A;

FIG. 1C is another perspective view of the tibial baseplate and supportstructure shown in FIG. 1A;

FIG. 1D is a bottom, plan view of the tibial baseplate and supportstructure shown in FIG. 1A;

FIG. 1E is an anterior, elevation view of the support structure shown inFIG. 1B;

FIG. 1F is a side, elevation view of the support structure shown in FIG.1B;

FIG. 1G is a top, plan view of the support structure shown in FIG. 1B;

FIG. 1H is an exploded, perspective view of knee prosthesis componentsmade in accordance with the present disclosure and suitable for use in atotal knee replacement surgery;

FIG. 2A is a perspective view of a relatively large-sized tibialbaseplate with a tibial baseplate support structure made in accordancewith the present disclosure attached thereto;

FIG. 2B is another perspective view of the tibial baseplate and supportstructure shown in FIG. 2A;

FIG. 2C is a side, elevation view of the tibial baseplate and supportstructure shown in FIG. 2A;

FIG. 2D is an anterior, elevation view of the tibial baseplate andsupport structure shown in FIG. 2A;

FIG. 2E is an anterior, elevation view of the tibial baseplate supportstructure shown in FIG. 2A;

FIG. 2F is a side, elevation view of the support structure shown in FIG.2E;

FIG. 2G is a bottom, plan view of the support structure shown in FIG.2E;

FIG. 2H is a side, elevation, section view of the tibial component andsupport structure shown in FIG. 2C;

FIG. 3A is a perspective view of a tibial baseplate with a tibialbaseplate support structure made in accordance with the presentdisclosure attached thereto, in which the support structure has ahemispherical medullary portion;

FIG. 3B is another perspective view of the tibial baseplate and supportstructure shown in FIG. 3A;

FIG. 3C is another perspective view of the tibial baseplate and supportstructure shown in FIG. 3A;

FIG. 3D is a side, elevation view of the tibial baseplate and supportstructure shown in FIG. 3A;

FIG. 3E is an anterior, elevation view of the tibial baseplate andsupport structure shown in FIG. 3A;

FIG. 3F is a bottom, plan view of the tibial baseplate and supportstructure shown in FIG. 3A;

FIG. 3G is a perspective view of the tibial baseplate support structureshown in FIG. 3A;

FIG. 3H is another perspective view of the support structure shown inFIG. 3G;

FIG. 4A is a side, elevation view of a tibial baseplate with a tibialbaseplate support structure made in accordance with the presentdisclosure attached thereto, in which the support structure has anangled proximal face;

FIG. 4B is a perspective view of the support structure shown in FIG. 4A;

FIG. 4C is a posterior, elevation view of the tibial baseplate andsupport structure shown in FIG. 4A;

FIG. 4D is a posterior, elevation view of the support structure shown inFIG. 4C;

FIG. 4E is an anterior, elevation view of the tibial baseplate andsupport structure shown in FIG. 4A;

FIG. 4F is an anterior, elevation view of the support structure shown inFIG. 4E;

FIG. 5A is a perspective view of a tibial baseplate support structurehaving a stepped platform portion in accordance with the presentdisclosure;

FIG. 5B is another perspective view of the support structure shown inFIG. 5A;

FIG. 5C is another perspective view of the support structure shown inFIG. 5A;

FIG. 5D is another perspective view of the support structure shown inFIG. 5A;

FIG. 5E is another perspective view of the support structure shown inFIG. 5A;

FIG. 5F is an anterior, elevation view of the support structure shown inFIG. 5A;

FIG. 6A is a perspective view of a support structure having anasymmetric, single-sided platform portion in accordance with the presentdisclosure;

FIG. 6B is another perspective view of the support structure shown inFIG. 6A;

FIG. 6C is another perspective view of the support structure shown inFIG. 6A;

FIG. 6D is another perspective view of the support structure shown inFIG. 6A;

FIG. 6E is another perspective view of the support structure shown inFIG. 6A;

FIG. 6F is a bottom, plan view of the support structure shown in FIG.6A; and

FIG. 6G is a top, plan view of the support structure shown in FIG. 6A.

Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplifications set out hereinillustrate exemplary embodiments of the present invention, and suchexemplifications are not to be construed as limiting the scope of theinvention in any manner.

DETAILED DESCRIPTION

The present disclosure provides a monolithic implant support structurewhich provides a stable implant mounting surface in a severely damagedor diseased bone. In the exemplary embodiments discussed below, thesupport structure provides a foundation for supporting a tibialbaseplate that is resistant to subsidence while also facilitatingreplacement and/or augmentation of metaphyseal or diaphyseal bone withinthe tibia. The support structure may be made of a porous bone ingrowthmaterial that provides a scaffold for bone ingrowth on multiplesurfaces. These surfaces present large, three-dimensional areas of boneingrowth material to the surrounding healthy bone for secure and stablelong term fixation of the support structure to the proximal tibia. Atibial baseplate may be mechanically attached to the support structure,which facilitates later removal of the tibial baseplate during arevision surgery while preserving the prosthesis foundation provided bythe support structure and ingrown bone.

A support structure in accordance with the present disclosure may beformed from a single piece of highly porous biomaterial. A highly porousbiomaterial is useful as a bone substitute and as cell and tissuereceptive material. A highly porous biomaterial may have a porosity aslow as 55%, 65%, or 75% or as high as 80%, 85%, or 90%, or may have anyporosity within any range defined by any of the foregoing values. Anexample of such a material is produced using Trabecular Metal®Technology generally available from Zimmer, Inc., of Warsaw, Ind.Trabecular Metal® is a trademark of Zimmer, Inc. Such a material may beformed from a reticulated vitreous carbon foam substrate which isinfiltrated and coated with a biocompatible metal, such as tantalum, bya chemical vapor deposition (“CVD”) process in the manner disclosed indetail in U.S. Pat. No. 5,282,861 to Kaplan, the entire disclosure ofwhich is expressly incorporated herein by reference. In addition totantalum, other metals such as niobium, or alloys of tantalum andniobium with one another or with other metals may also be used.

Generally, the porous tantalum structure includes a large plurality ofligaments defining open spaces therebetween, with each ligamentgenerally including a carbon core covered by a thin film of metal suchas tantalum, for example. The open spaces between the ligaments form amatrix of continuous channels having no dead ends, such that growth ofcancellous bone through the porous tantalum structure is uninhibited.The porous tantalum may include up to 75%, 85%, or more void spacetherein. Thus, porous tantalum is a lightweight, strong porous structurewhich is substantially uniform and consistent in composition, andclosely resembles the structure of natural cancellous bone, therebyproviding a matrix into which cancellous bone may grow to providefixation of the support structure to the patient's bone.

The porous tantalum structure may be made in a variety of densities inorder to selectively tailor the structure for particular applications.In particular, as discussed in the above-incorporated U.S. Pat. No.5,282,861, the porous tantalum may be fabricated to virtually anydesired porosity and pore size, and can thus be matched with thesurrounding natural bone in order to provide an improved matrix for boneingrowth and mineralization.

Various configurations and sizes for a support structure arecontemplated in accordance with the present disclosure. Exemplaryconfigurations are provided in the description below and associateddrawings. For example, FIGS. 1A-1G illustrate a monolithic (or“monoblock”) support structure for a relatively small size tibialbaseplate and a correspondingly small medullary defect. In FIGS. 2A-2H,a second monolithic support structure similar to the support shown inFIGS. 1A-1G is illustrated, but is adapted for a larger size tibialbaseplate and a correspondingly larger medullary defect. FIGS. 3A-3Hillustrate yet another monolithic support structure with a medullaryportion having an alternative geometrical configuration, namely, ahemispherical configuration.

Turning now to FIG. 1A, a nominally small-sized baseplate supportstructure 10 is shown mated to a correspondingly small nominal sizetibial baseplate 12. Support structure 10 includes proximal platform 14,which mechanically attaches to the distal surface of tibial baseplate 12(as described below) and has an outer periphery substantially matchingthe outer periphery of baseplate 12, which in turn has a peripheryshaped to correspond with a proximal resected surface T_(S) of ananatomic tibia T (FIG. 1H). Medullary portion 16 extends distally fromdistal surface 34 of platform 14, and is integrally, monolithicallyformed with platform 14 such that support structure 10 is formed from asingle, monolithic piece of material.

In the illustrated embodiments discussed below, support structure 10 isformed from bone ingrowth material, such as porous tantalum as describedabove, which provides a scaffold for the ingrowth and interdigitation ofbone with both platform 14 and medullary portion 16 of support structure10. As such ingrowth occurs over time, support structure 10 becomesintegrally formed with the tibia to provide a stable, bone-like supportfoundation for tibial baseplate 12. Advantageously, as noted below, thissupport foundation may remain in place even through a revision surgeryto replace tibial baseplate 12 with a new tibial baseplate.

Medullary portion 16 is generally conically shaped, as described indetail below, and includes opening 22 through which baseplate keel 18may pass. In the illustrative embodiment, medullary portion 16 has asubstantially closed peripheral profile, such that keel 18 is surroundedby opening 22. As most clearly shown in FIG. 1B, baseplate keel 18includes a pair of fins 20 extending between the distal end of keel 18and the distal surface of the tibial baseplate 12. To accommodate fins20, opening 22 includes flared cutouts 24 extending therethrough.Cutouts 24 interrupt the otherwise conical or cylindrical shape ofopening 22, and selectively expand the periphery of opening 22 along thelongitudinal extent of medullary portion 16 to provide a space sized tofit fins 20 with clearance. As cutouts 24 extend toward the proximal endof opening 22 (at proximal surface 26, as shown in FIG. 1G), cutouts 24flare outwardly to accommodate the corresponding outward flare of fins20 as they extend toward their junction with platform portion 30 oftibial baseplate 12.

In the illustrated embodiment of FIGS. 1C and 1D, baseplate fins 20 donot protrude outwardly beyond the outer periphery of medullary portion16. In order to maintain a desired minimum material thickness throughoutsupport structure 10, however, fin windows 28 are formed in the materialof support structure 10. In an exemplary embodiment, the minimummaterial thickness in support structure is at least 1 mm. Fin windows 28span an area from a distal window end, at which fins 20 are sufficientlyproximate to medullary portion 16 to prevent the desired minimummaterial thickness from being achieved, to a proximal window end atdistal surface 34 of platform portion 14. Thus, fin windows 28 spanjunction 32 formed between medullary portion 16 and platform 14 ofsupport structure 10.

The size of the interruptions in junction 32 caused by fin windows 28 isminimized in order to maximize the strength of junction 32 betweenmedullary portion 16 and platform 14. At the same time, the size ofwindows 28 is made sufficiently large to maintain at least a minimumdesired clearance between keel 18 and the interior surface defined byopening 22 of medullary portion 16. Junction 32 is also radiused toprevent stress concentrations within the material of support structure10 during in vivo prosthesis use.

As best seen in FIGS. 1C and 1D, clearance is provided between theperipheral wall of opening 22 formed through medullary portion 16 ofsupport structure 10, and the outer peripheral wall of tibial baseplatekeel 18. This clearance ensures a smooth passage of keel 18 throughopening 22 upon assembly of baseplate 12 to support structure 10, andfurther ensures that the orientation of tibial baseplate 12 with respectto support structure 10 after such assembly is dictated solely by theinterface between platform 14 of support structure 10 and platformportion 30 of tibial baseplate 12. The clearance between keel 18 andmedullary portion 16 may be the same as clearance 148 between keel 118and medullary portion 116 of larger-sized support structure 110, asillustrated in FIG. 2H and described below.

In the illustrative embodiment of FIGS. 1A-1G, medullary portion 16 ofsupport structure 10 has a truncated, generally conical outer surface.As shown in FIGS. 1E and 1F, the truncated cone defined by medullaryportion 16 defines a central axis A₁. When viewed from differentaspects, this truncated cone defines varying degrees of taper withrespect to axis A₁. Referring to FIG. 1E, for example, the tapersdefined by the medial and lateral boundaries of medullary portion 16(i.e., the taper when viewed from an anterior or posterior perspective)defines taper angle α with respect to axis A₁. On the other hand,referring to FIG. 1F, the posterior boundary of medullary portion 16defines taper angle β, and no taper is defined by the anterior boundaryof medullary portion 16. Thus, while the present disclosure refersgenerically to truncated “cone shaped” augments and portions ofaugments, it is contemplated that such cone shapes need not be strictlyconical, but can have varying cross-sectional geometries such as oval,elliptical, or any other non-circular cross-section.

It is contemplated that the taper angles defined by support structure 10may have a variety of nominal values or combinations of nominal values.For example, the medial and lateral taper angles α (FIG. 1E) may beequal (as shown) or different, and may define any taper angle. In oneexemplary embodiment, angle α is as little as 4 degrees or 9.5 degreesand as large as 12 degrees or 15 degrees, or may be any value within anyrange defined by any of the foregoing values. Posterior taper angle βmay be as little as 10 degrees or 12 degrees, or as large as 17 degreesor 19 degrees, or may be any value within any range defined by any ofthe foregoing values. In this exemplary embodiment, no anterior taperangle is defined by medullary portion 16, i.e., the anterior edge ofmedullary portion 16 is substantially parallel to axis A₁ as viewed fromthe sagittal perspective of FIG. 1F. However an anterior taper angle maybe provided as desired or required for a particular application, suchthat the anterior edge of medullary portion 16 converges toward axis A₁along a proximal-to-distal direction. Additional discussion of exemplarytaper angles for medullary portion 16, in the context of a separatelyformed conically-shaped augment, appears in U.S. patent application Ser.No. 12/886,297, incorporated by reference above.

As best shown in FIGS. 1C and 1E, distal surface 34 of platform 14defines a substantially flat, planar surface adapted to mate with asimilarly planar resected surface of a proximal tibia. However, asillustrated in FIG. 1E, it is contemplated that distal surface 34 may be“stepped” such that one of the medial and lateral side of supportstructure 10 is thicker than the other side. This thicker side 36defines stepped distal surface 34′, which is positioned to rest upon aportion of the tibia which has been more deeply resected than the other,adjacent portion of the resected tibia. Such a “stepped” configurationmay prevent unnecessary removal of healthy bone in an asymmetric defect.More particularly, a surgeon may avoid resection of healthy bone stockon one side of the tibial plateau where no augmentation is required,while resecting damaged bone from the other side where more severetrauma and/or degradation has occurred.

Another embodiment including a stepped configuration of the distalsurface of the augment platform is illustrated in FIGS. 5A-5F. Supportstructure 410 is similar to support structure 10 described above, andreference numbers in FIGS. 5A-5F refer to analogous structures describedabove with respect to support structures 10. However, platform portion414 includes a thicker side 436 on the opposite side of supportstructure 410 as compared to thicker side 36 of support structure 10.Distal surface 434′ is offset distally from distal surface 434 in asimilar fashion as described above. At the periphery of platform 414,however, thicker side 436 includes tapered portion 415, similar totapered portion 115 described below with respect to larger-sized supportstructure 110 (FIGS. 2A-2H). Holes 446 are also modified in supportstructure 410 to include shoulders 447 therein. Shoulders 447 are sizedand adapted to engage shoulder 43 of nut 42 (FIG. 1A) to axially capturenut 42 within holes 446.

Another embodiment including an asymmetric platform configuration isillustrated in FIGS. 6A-6G. Support structure 510 is similar to supportstructure 10 described above, and reference numbers in FIGS. 6A-6G referto analogous structures described above with respect to supportstructures 10. However, only one side of platform portion 514 isprovided, such that support structure is only designed to replace eithermedial or lateral defects in the proximal tibia while leaving the otherside un-augmented. In the illustrated embodiment, support structure 510can be used to replace defects in the lateral tibial plateau when usedin a left knee, or in the medial tibial plateau when used in a rightknee. It is also contemplated that a similar, substantially mirror-imagesupport structure may be provided for augmentation of bone resected toremove left-medial and right-lateral defects. In another alternative,the overall shape of the augment may be asymmetric, such that individualasymmetric component geometries may be provided for each ofright-lateral, right-medial, left-lateral and left-medial defects. Likeholes 446 of support structure 410, holes 546 include shoulders 547.

In addition to the various geometrical arrangements of platforms 14,414, 514 described above, it is contemplated that medullary portion 16may have an extended axial length on one side, as represented byextended axial portion 38 of medullary portion 16 in FIG. 1E. Similar tothicker side 36, this additional axial length on one side allows asurgeon to correct an asymmetric medullary defect without removal ofhealthy bone on a side of the bone lacking such defects.

Referring back to FIG. 1A, support structure 10 mounts to tibialbaseplate 12 via externally threaded fastener 40 and a correspondinginternally threaded nut 42. Platform portion 30 of tibial baseplate 12includes a plurality of countersunk holes 44 sized to receive the headof fastener 40. Upon assembly, fastener 40 passes through holes 44 andinto correspondingly formed holes 46 formed in platform 14 of supportstructure 10 (FIG. 1B), which are aligned with holes 44 when supportstructure 10 and baseplate 12 are coupled to one another. Nut 42 isreceived within holes 46, such that fastener 40 may be threadablyengaged with nut 42. Shoulder 43 axially fixes nut 42 with respect todistal surface 34 of support structure 10, so when fastener 40 istightened, fastener 40 and nut 42 cooperate to mechanically fastensupport structure 10 to tibial baseplate 12. Fasteners 40, nut 42 andtibial baseplate 12 may be made of a biocompatible material, such astitanium or cobalt chrome molybdenum. Such mechanical fixationfacilitates revision surgeries by allowing tibial baseplate 12 to beremoved from support structure 10 by disengaging fasteners 40 fromrespective nuts 42. Thus, even when support structure 10 has becomeembedded within the tibia over time via bone ingrowth into supportstructure 10, tibial baseplate remains removable without removal ofsupport structure 10 or the surrounding bone.

FIG. 1H illustrates the use of support structure 10 and tibial baseplate12 in conjunction with other prosthesis components used in a total kneereplacement (TKR) surgical procedure. In particular, femoral component50 may be provided for implantation upon femur F, in order to replacethe articular surfaces of the natural femoral condyles with prostheticcondyles 52, 54. Femur F may be prepared to receive femoral component 50by resection of the femoral condyles to create femoral facets F_(F),which are positioned and configured to abut the corresponding facets ofbone-contacting surface 56 of femoral component 50.

Tibial bearing component 58 may be fitted to tibial baseplate 12 inorder to provide a low-friction articular interface with condyles 52, 54of femoral component 52. In one exemplary embodiment, tibial bearingcomponent 58 cooperates with tibial baseplate 12 to form a “fixedbearing” design in which tibial bearing component 58 is immovablyaffixed to tibial baseplate 12 upon implantation. In another exemplaryembodiment, tibial bearing component 58 is a “mobile bearing” design inwhich tibial bearing component is slidably and/or rotatably movable withrespect to tibial baseplate 12 during knee articulation.

Tibial baseplate 12 and support structure 10 are affixed to tibia T uponprosthesis implantation. In one embodiment, the anatomic articularsurfaces of tibia T are resected to create a substantially planarresected surface T_(S), which is configured to abut the substantiallyplanar distal surface 34 of support structure 10. A resected, generallyconical cavity is also formed in tibia T to correspond with medullaryportion 16 of support structure 10. An exemplary apparatus and methodfor forming medullary portion 16 is disclosed in U.S. provisional patentapplication Ser. No. 61/522,872 filed Aug. 12, 2011 and entitledPROSTHESIS RESECTION GUIDE, the entire disclosure of which is herebyexpressly incorporated herein by reference.

Turning now to FIGS. 2A and 2B, a relatively larger nominal size ofsupport structure 110 is shown mated to a corresponding larger nominalsize of tibial baseplate 112. Support structure 110 and baseplate 112are generally similar to the small nominal size support structure 10 andcorrespondingly small tibial baseplate 12, discussed in detail above,but are larger in dimension to accommodate larger natural anatomies.Reference numbers in FIGS. 2A-2H refer to analogous structures describedabove with respect to support structures 10.

Referring to a comparison of FIGS. 1E and 2E, for example, large sizesupport structure 110 defines an overall width W_(L) of platform 114that is larger than the corresponding width W_(S) of platform 14. Thedistal end of medullary portion 116 defines a medial-lateral diameterDML_(L) which is also correspondingly larger than medial-lateraldiameter DML_(S) of the relatively smaller medullary portion 16 ofsupport structure 10. It should be appreciated that the term “diameter”as used herein does not necessarily imply a round cross-section, but mayalso refer to a dimension across a non-round cross section. For example,a diameter may be the major or minor axes of an ellipse, oval or otheroblong shape.

Turning to a comparison of FIGS. 1F and 2F, large size support structure110 defines an overall height H_(L), and anteroposterior length L_(L) ofplatform 114, and an anteroposterior diameter DAP_(L) of medullaryportion 116 that are larger than the corresponding height H_(S), lengthL_(S), and medullary portion diameter DAP_(S) of the relatively smallersupport structure 10. Smaller and larger sized support structures 10,110 each define angle β between platforms 14, 114 and the posteriorportion of medullary portions 16, 116, respectively. It is contemplatedthat angles α, β and θ may be different among different supportstructure configurations.

Moreover, small size support structure 10 is generally adapted for asmall size tibia and a relatively small medullary defect within thetibia, which is filled in by medullary portion 16 as described in detailbelow. Larger size support structure 110, on the other hand, is adaptedfor a larger tibia having a relatively large volume of defective bonewithin the tibia. However, it is contemplated that any size platform maybe paired with any size medullary portion. In an exemplary embodiment, afamily or kit of support structures may be provided with differingsupport structure size/geometry combinations. Each individual supportstructure may be suitable for one of a wide range of natural tibia sizesand bone defect geometries.

In an exemplary embodiment, support structure dimensions may be any ofthe following values, or may be any value within any range defined bythe following values: support structure height H_(S) may be as little as2 mm, 5 mm or 10 mm, while height H_(L) may be as much as 60 mm, 80 mmor 100 mm; support structure length L_(S) may be as little as 40 mm, 50mm or 60 mm, while length L_(L) may be as much as 90 mm, 110 mm or 130mm; anteroposterior diameter DAP_(S) may be as little as 10 mm, 30 mm or50 mm, while anteroposterior diameter DAP_(L) may be as much as 60 mm,80 mm or 100 mm; support structure width W_(S) may be as little as 50mm, 60 mm or 70 mm, while width W_(L) may be as much as 90 mm, 110 mm or130 mm; medial-lateral diameter DML_(S) may be as little as 10 mm, 30 mmor 50 mm, while medial-lateral diameter DML_(L) may be as much as 90 mm110 mm or 130 mm; support structure thickness T_(S) may be as little as1 mm, 3 mm or 5 mm, while thickness T_(L) may be as much as 20 mm, 25 mmor 30 mm. An overall height of medullary portions 16, 116 may bedetermined by subtracting thickness T_(S), T_(L) from overall structureheight H_(S), H_(L) respectively.

Larger size support structure 110, in addition to having larger nominaldimensions as detailed above, may also have certain unique geometricalcharacteristics. For example, referring to FIG. 2A, platform 114includes tapered portion 115, which may taper in overall width andanteroposterior length in a similar fashion to a natural proximal tibia.Tapered portion 115 is beneficial when a large amount of the naturalproximal tibia is resected, thereby requiring a large thickness T_(L) ofplatform 114 to maintain the natural joint line of the knee (asdiscussed above). In addition, referring to FIGS. 2B and 2C, fin windows128 are substantially smaller than the corresponding fin windows 28 onsmaller sized support structure 10, because the larger size of supportstructure 110 allows for a minimum material thickness to be maintainedthrough more of medullary portion 116 while providing adequate clearance148 (FIG. 2H) for baseplate fins 120 of larger baseplate 112.

Medullary portions 16, 116 define a truncated, generally conical shape,as described in detail above. However, it is also contemplated that themedullary portion of a support structure in accordance with the presentdisclosure may have any shape, as required or desired for a particularapplication.

Turning to FIG. 3A, for example, hemispherical support structure 210includes platform 214, which may be similar to platforms 14 or 114described above, and hemispherical medullary portion 216. Referencenumbers in FIGS. 3A-3H refer to analogous structures described abovewith respect to support structures 10.

In the illustrative embodiment of FIG. 3A, hemispherical medullaryportion 216 includes cutouts 228 in place of fin windows 28, 128 toaccommodate fins 220 of tibial baseplate 212, though it is contemplatedthat the size of hemispherical medullary portion 216 may be expanded tocreate a window similar to fin windows 28, 128, or may be furtherexpanded to eliminate the need for any fin accommodating window whilemaintaining a minimum desired material thickness.

As described below, hemispherical medullary portion 216 may be matedwith a correspondingly hemispherical cavity created within the tibia.Advantageously, such a hemispherical tibial cavity may be created withstandard instruments typically used to prepare the acetabular cavity ofa hip to receive an acetabular cup. Such instruments may includeacetabular reamers, which are available in sizes small enough to be usedwith a tibia, such as a diameter as small as 18-20 mm, for example.Spherical support structure 210 may be provided in a wide range of sizesand geometrical configurations to accommodate a correspondingly widerange of anatomical configurations.

It is also contemplated that a support structure in accordance with thepresent disclosure may have other variations in geometry. For example,referring to FIGS. 4A-4F, support structure 310 is illustrated withproximal surface 326 defining an angled profile with respect to axis A₁(FIG. 4A). Reference numbers in FIGS. 4A-4F refer to analogousstructures described above with respect to support structures 10.

Angled proximal surface 326 accommodates tibial baseplate 312, which issimilarly angled. To create angled proximal surface 326 withoutdisturbing the geometry of medullary portion 316 (which, in theillustrated embodiment, is substantially similar to medullary portion 16of support structure 10), the thickness of platform portion 314 isvaried rather than remaining constant (as thicknesses T_(S), T_(L) do asdescribed above). Thus, proximal surface 326 defines angle γ (FIG. 4A)with respect to axis A₁. The specific value of angle γ may varydepending on the corresponding angle of proximal portion 230 of tibialbaseplate 312, which in turn varies as a function of the chosenanteroposterior angle of the proximal tibial resection performed by asurgeon (i.e., the “tibial slope”). In exemplary embodiments, angle γmay be as little as 0, 3, or 5 degrees, or as large as 7, 10, or 15degrees, or may be any value within any range defined by the foregoingvalues. In these exemplary embodiments, angle γ is positive whenproximal surface 326 corresponds to a positive tibial slope, which is aslope angled upward along a posterior-to-anterior direction.

In use, one of support structures 10, 110, 210, 310 is implanted uponthe proximal tibia when it is determined that portions of the proximaltibial plateau and the metaphyseal and/or diaphyseal bone within thetibia are both damaged and/or diseased, therefore requiring resection.Such bone is resected in a conventional manner, typically with referenceto the medullary canal. Additional instrumentation, known to personshaving ordinary skill in the art of knee arthroplasty and otherorthopaedic surgeries, may be used to orient tibial keel 18, 118, 218 or318 with respect to the medullary canal of the tibia, such that tibialbaseplate 12, 112, 212 or 312 will be properly centered on the resectedproximal surface of the tibia when the surgical implantation iscomplete.

In the metaphyseal and/or diaphyseal portions of the tibia a void iscreated in the bone to correspond to the geometry of the medullaryportion of the chosen support structure (i.e., one of medullary portions16, 116, 216, 316). In the case of a generally conical medullaryportion, such as medullary portions 16, 116, 316, a combination ofburrs, mills and/or reamers may be used to create a correspondinglyconical medullary void. In the case of a hemispherical medullaryportion, such as medullary portion 216, an appropriately sizedhemispherical reamer, similar to an acetabular reamer used in hiparthroplasty procedures, may be used to prepare the medullary void. Inall cases, the medullary void may be sized for a press-fit of medullaryportion 16, 116, 216 or 316, thereby preventing the need for bone cementto aid in the fixation of support structure 10, 110, 210 or 310 to thetibia. As noted above, ingrowth of natural bone into the material of thesupport structure may be the primary or sole method of fixation betweenthe tibial bone and support structure.

Support structures 10, 110, 210, 310 may be used to restore the jointline of the natural knee where a large amount of the proximal tibia hasbeen resected to remove correspondingly large amounts of diseased,damaged or otherwise defective bone stock. The combination of platforms14, 114, 214, 314 into a single monolithic structure with medullaryportions 16, 116, 216, 316, respectively, ensures that this joint lineis maintained over a long period of time by providing a largebone-contacting surface area. In addition, this monolithic combinationpresents many bone-contacting faces, each of which are oriented in adifferent direction with respect to the others to yield a“3-dimensional” or multi-faceted profile of bone-contacting faces. This3-dimensional profile facilitates multidirectional stabilization of thesupport structure, and of the tibial baseplate mounted thereto, therebyminimizing or eliminating subsidence, anteroposterior movement andmedial-lateral movement of the tibial prosthesis in vivo. Moreover, ithas been found that the stability provided by a monolithic supportstructure made in accordance with the present disclosure providesgreater stability than would otherwise be provided by a separate tibialcone-shaped implant and a plate-shaped tibial augment implant, whetherused in combination or alone.

Advantageously, a support structure made in accordance with the presentdisclosure does not require the use of cement for fixation to a bone.This lack of cement facilitates bone ingrowth by allowing bone tointerdigitate more deeply with the porous bone contacting surfaces ofplatforms 14, 114, 214, 314 and medullary portions 16, 116, 216, 316.This deep bone ingrowth provides stronger and more secure fixation thancould be expected from adhesion between bone cement and bone. Thus, asupport structure in accordance with the present disclosure provides abone replacement and restoration mechanism which gives rise to a stable,bone-like support structure for tibial baseplate components and otherassociated knee prosthesis components.

Also advantageously, tibial baseplates 12, 112, 212, 312 are removablefrom support structures 10, 110, 210 in a revision surgery, even ifsubstantial bone ingrowth has occurred between the tibia and supportstructures 10, 110, 210 or 310. Because no cement is required, asdiscussed above, cemented fixation between a tibial baseplate and asupport structure in accordance with the present disclosure is notrequired. Rather, mechanical fixation may be used, such as with fastener40 and nut 42 as detailed above. If a revision surgery is required, suchmechanical fixation can be reversed by removing fastener 40 from nut 42,thereby freeing the tibial baseplate from the support structure. Thesupport structure can be left behind, and may therefore remainthoroughly interdigitated with ingrown bone. This remaining supportstructure obviates the need for removal of any further bone stock duringa revision surgery, and provides a reusable, stable and strong supportplatform for a new tibial baseplate and/or other knee prosthesiscomponents.

Further, the strength of fixation between a support structure inaccordance with the present disclosure and the adjacent bone isunexpectedly stronger than other designs adapted for use without bonecement. The monolithic, integral nature of support structures 10, 110,210, 310 results in a stronger implant as compared to two separateimplants separately affixed to the bone. Thus, the overall area of boneingrowth for support structures 10, 110, 210, 310 is substantiallylarger than any other similarly sized individual tibial augmentstructure.

While the disclosure has been described as having exemplary designs, thepresent disclosure can be further modified within the spirit and scopeof this disclosure. This application is therefore intended to cover anyvariations, uses or adaptations of the disclosure using its generalprinciples. Further, this application is intended to cover suchdepartures from the present disclosure as come within known or customarypractice in the art to which this disclosure pertains and which fallwithin the limits of the appended claims.

1.-20. (canceled)
 21. A proximal tibial implant system, comprising: aproximal tibial implant including a tibial baseplate with a proximalbaseplate surface and a distal baseplate surface with a keel extendingtherefrom, said keel having a length extending distally from said distalbaseplate surface, said proximal tibial implant including at least afirst fin extending down a side of the keel substantially along thelength of the keel from the distal baseplate surface toward a distal endof the keel; and a support structure for use in conjunction with theproximal tibial implant, the support structure comprising: a platformhaving a proximal surface and a distal surface defining a platformthickness therebetween, said proximal surface and said distal surfacecooperating to define a platform outer periphery shaped to correspondwith a periphery of a resected proximal tibia, said periphery dividedinto a medial side and an opposing lateral side, said platform outerperiphery defining a platform medial-lateral width and a platformanteroposterior length; and a medullary portion extending distally fromsaid distal surface of said platform and from at least one of saidmedial side and said lateral side, said medullary portion monolithicallyformed with said platform and comprising: a medullary portionanteroposterior diameter less than said platform anteroposterior length;a medullary portion medial-lateral diameter less than said platformmedial-lateral width; and a medullary portion height measured along aproximal/distal extent of said medullary portion, wherein the supportstructure provides a longitudinal passage that extends through thesupport structure from a proximal end opening in the proximal surface ofthe platform toward a distal end of the medullary portion, said keelpositioned in the longitudinal passage.
 22. The proximal tibial implantsystem of claim 21, wherein: said medullary portion defines a centrallongitudinal axis extending along the proximal/distal extent thereof;and said platform thickness is variable across at least one of saidplatform medial-lateral width and said anteroposterior length, such thatsaid proximal surface of said platform defines an angle with saidcentral longitudinal axis of said medullary portion.
 23. The proximaltibial implant system of claim 21, wherein said monolithically formedplatform and medullary portion are formed with a single piece of porousbone-ingrowth material.
 24. The proximal tibial implant system of claim21, wherein the longitudinal passage extends to a distal end opening atthe distal end of the medullary portion, and wherein said keel protrudesfrom said distal end opening.
 25. The proximal tibial implant system ofclaim 24, wherein said first fin protrudes from said distal end opening.26. The proximal tibial implant system of claim 21, wherein themedullary portion includes a side wall with an interior surface situatedalong said longitudinal passage and an exterior surface opposite saidinterior surface for contacting bone upon implantation, the supportstructure providing at least a first open window that extends throughthe side wall from said interior surface to said exterior surface. 27.The proximal tibial implant system of claim 26, wherein said first finis received in said first open window with a clearance space existingbetween said first fin and said first open window.
 28. The proximaltibial implant system of claim 26, wherein said first fin extendsthrough said first open window so as to protrude from the exteriorsurface of said side wall.
 29. The proximal tibial implant system ofclaim 26, wherein the first open window includes a distal window endthat terminates proximal of a distal end of the medullary portion. 30.The proximal tibial implant system of claim 21, wherein the medullaryportion defines a hemispherical shape.
 31. The proximal tibial implantsystem of claim 21, wherein said platform thickness is substantiallyconstant across said platform medial-lateral width and saidanteroposterior length.
 32. A support structure for use in conjunctionwith a proximal tibial implant, the support structure comprising: aplatform having a proximal surface and a distal surface defining aplatform thickness therebetween, said proximal surface and said distalsurface cooperating to define a platform outer periphery shaped tocorrespond with a periphery of a resected proximal tibia, said platformouter periphery defining a platform medial-lateral width and a platformanteroposterior length; and a medullary portion extending distally fromsaid distal surface of said platform, said medullary portionmonolithically formed with said platform and comprising: a medullaryportion anteroposterior diameter less than said platform anteroposteriorlength; a medullary portion medial-lateral diameter less than saidplatform medial-lateral width; and a medullary portion height measuredalong a proximal/distal extent of said medullary portion, wherein saidmedullary portion defines a central longitudinal axis extending alongthe proximal/distal extent thereof, and wherein said platform thicknessis variable across at least one of said platform medial-lateral widthand said anteroposterior length such that said proximal surface of saidplatform defines an angle with said central longitudinal axis of saidmedullary portion.
 33. The support structure of claim 32 in combinationwith a tibial baseplate having a proximal baseplate surface and a distalbaseplate surface with a keel extending therefrom, said distal baseplatesurface configured to face the proximal surface of said platform withsaid keel extending through an opening formed in said medullary portion.34. The combination of claim 33, wherein said proximal baseplate surfaceand said distal baseplate surface cooperate to define a tibial baseplateperiphery corresponding to said platform outer periphery.
 35. A supportstructure for use in conjunction with a proximal tibial implant, thesupport structure comprising: a platform having a proximal surface and adistal surface defining a platform thickness therebetween, said proximalsurface and said distal surface cooperating to define a platform outerperiphery shaped to correspond with a periphery of a resected proximaltibia, said platform outer periphery defining a platform medial-lateralwidth and a platform anteroposterior length; and a medullary portionextending distally from said distal surface of said platform, saidmedullary portion monolithically formed with said platform andcomprising: a medullary portion anteroposterior diameter less than saidplatform anteroposterior length; a medullary portion medial-lateraldiameter less than said platform medial-lateral width; a medullaryportion height measured along a proximal/distal extent of said medullaryportion; and at least a first open window in a side wall of themedullary portion, wherein the first open window spans a junction formedbetween said medullary portion and said platform.
 36. The supportstructure of claim 35 in combination with a tibial baseplate having aproximal baseplate surface and a distal baseplate surface with a keelextending therefrom, said proximal baseplate surface and said distalbaseplate surface cooperating to define a tibial baseplate peripherycorresponding to said platform outer periphery, said keel extendingthrough an opening formed in said medullary portion.
 37. The combinationof claim 36, wherein said tibial baseplate comprises at least a firstfin extending radially outwardly from said keel, said first open windowsized and positioned to provide clearance for said first fin of saidtibial baseplate.
 38. The support structure of claim 35, wherein saidfirst open window maintains a minimum thickness of the material of saidsupport structure of at least 1 mm.
 39. The support structure of claim35, wherein said monolithically formed platform and medullary portionare formed with a single piece of porous bone-ingrowth material.
 40. Thesupport structure of claim 35, wherein the first open window includes adistal window end that terminates proximal of a distal end of themedullary portion.